The present invention relates to gyrometers without material rotor, of the type employing the influence of the Coriolis forces on the particles of a fluid contained in a resonant cavity, and comprising excitation means for generating and maintaining longitudinal acoustic oscillations in said cavity along an axis of excitation, and means for measuring the transverse pressure variations appearing at the level of a vibration node, along an axis of measurement, when the cavity pivots about an axis perpendicular to the plane of the oscillations.
In known devices of this type, such as the acoustic gyrometer described in U.S. Pat. No. 2,999,389, an electric oscillation generator causes a diaphragm, disposed at the bottom of a closed cavity, to vibrate via a trip coil. A longitudinal acoustic wave is established in the cavity. As a function of the dimensions of the cavity and of the excitation frequency, the acoustic wave is arranged to present a pressure node at a well determined point. When the cavity pivots about an axis perpendicular to the plane of the oscillations, the Coriolis forces generate a stationary oscillatory mode, transverse to the excitation mode, of which the amplitude is proportional to the speed of rotation of the cavity. To measure this transverse acoustic pressure, which represents the amplitude of the transverse oscillation, two tubes are disposed on either side of the cavity on an axis of measurement, located at the level of the pressure mode, to transmit the variations in pressure to a differential microphone.
It is admitted that the speed of rotation of the cavity may be deduced from the following relationship: EQU F=2 p.OMEGA..mu..multidot. sin .omega..sub.1 t
wherein p is the mass of the gaseous molecules,
.mu.. the amplitude of the instantaneous sinusoidal speed PA1 .OMEGA.the angular speed of rotation of the card, PA1 .omega..sub.1 the frequency of oscillation of the molecules, PA1 F the amplitude of the transverse pressure variations due to the Coriolis forces. PA1 either by using an exciter constituted by a piezoelectric disc of very small diameter with respect to the dimensions of the cavity. In that case, spherical waves are generated; PA1 or by using an exciter of large diameter but of very small thickness, of the order of some microns. In that case, flat waves are generated.
of excitation of the molecules,
However, in practice, such a gyrometer presents drawbacks:
It is delicate to produce since the half wave length of the acoustic excitation wave must correspond exactly to the length of the cavity, failing which the position of the pressure node, on which the measurements are effected, will be imprecise.
The influence of the harmonics is not negligible and is translated by errors in measurements.
The coupling between the excitation member and the gas is important, which brings about a low quality factor, which is the ratio of the resonant frequency to the bandwidth, of the resonant cavity and a considerable influence of the defects of the exciters on the exact location of the pressure node; the use of a differential microphone connected to the cavity by pressure pick-up tubes creates phase-shifts due to the pneumatic delays; in addition, the coupling with the cavity is not negligible due to the size of the diameters of the necessary pipes with respect to the wave-length of the oscillations.